Ringer

Ringer is a term with several meanings:

An ornithologist trained in bird ringing is called a ringer.
In colloquial English language, it refers to a specialist who is clandestinely brought into a group or team to bolster its capabilities. (See the original M*A*S*H movie, in which the character, Capt. Oliver Harmon 'Spearchucker' Jones played by Fred Williamson, a former professional American football player, is used as the secret weapon: He is described several times as the 'ringer'.)
In Australian English vernacular and folklore, the speed-shearer in the wool shed is the ringer. (See the Australian song, 'Click go the Shears'.)
In Tolkien fandom, a ringer is a fan of J.R.R. Tolkien novels and characters.
In Role playing games, a ringer is a player character who is a direct or near-direct copy of a character from a work of popular fiction (derived from "dead ringer").
In the game of horseshoe pitching, a ringer is a shoe that encircles the stake and is counted as 3 points.
A ringer is a style of T-shirt with a different color of fabric around the neck and sleeves.
The Ringer is a 2005 comedy film starring Johnny Knoxville and Katherine Heigl.
A ringer is a type of crane mounted on a large ring, usually of very high capacity.
A ringer is a stolen car whose vehicle identification number plate has been replaced from a car of the same type which had been written off in a car accident.